Electroplating of articles with chromium



Patented 25, 1934 UNITED STATES PATENT oF cE ELECTROPLATING or ARTICLES wrrn cnnomum Oscar Bornhauscr,

Strasbourg (Bas Bhin France, assignor to Socit d'Electrochimie, dElectromctallurgie et des Aclerles wElectriques dUgine, Paris, France, a corporation of France No Drawing. Application July 28, 1933, Serial No.682,707. InFrancc'July 29,1932

'1' Claims. (01. 204-1) square decimetre cannot be exceeded, the normal mean current density seldom exceeding amperes per square decimetre. By reason of the low yield per ampere which can be obtained, it is necessary to provide for a lengthy duration of working-in order to obtain chromium coatings of useful thickness. Moreover, in places where excessive current densities prevail, the quality of the coating is jeopardized.

It has indeed been attempted to improve chromium plating baths by means of the most varied additions. With this object bichromates have been used, for example, but without obtaining decisive advantages; moreover, the quantities. of added substances and their proportions with respect to the chromic acid are so variable that it is impossible to establish any theoretical basis for the process.

The present invention is based on methodical researches which have resulted in the establishment of the fact that considerable modifications of current density conditions may be rendered possible by adding to the chromic acid (between very close limits the reason for which will be given below) ions of Na (in the form of Na: Cr: 01, or of an equivalent quantity of neutral sodium chromate or sodium hydroxide). In this manner the maximum admissible current density can be raised to a value ten times greater than the known value. The quality of the layer of chromium obtained is, moreover, notably improved; it has an almost yellowish coloration, a remarkable ductility and a total absence of structure.

This effect is produced when the composition of the chromium plating bath corresponds to conditions which ensure in this bath the presence of sodium tetrachromate. that is to say, the salt of tetrachromic acid H2 Cr401a=H2 C1O4.3C1O3.

It is known'from the literature dealing with this matter that the formation of sodium tetrachromate in concentrated solutions is certainly produced when the content of CrOs reaches 67%.

and that of NazO 10%. When NazO-is present in excess of 3.5% the tetrachromate no longer exists. An excess of C10; does not cause it to disappear but merely limits the quantitative content of the solution in tetrachromate.

When the solutions are more dilute, it is necessary that there should be a slight excess of chromic cid with respect to the normal composition res ting from the formula, for reasons of equilibrium; this excess is the greater the weaker the solution.

During the electrolysis everything takes place as if onewere working in the presence of a mixture of sodium tetrachromate and chromic acid.

It has been possible by experiment to determine that an equally high yield is obtained between the following limits in the proportions of NaOH and CrOa:

2 mol. NaOH to 4 mol; CrOa on the one hand and 2 mol. NaOH to 6 mol. CrO: on the other hand.

The meanratio is thus: 2 mol. NaOH to 5 mol.

CrOs.

While the composition of the tetrachromate is: 2 mol. NaOH to 4 mol. 'C1O3.

This proportion is to be maintained for the saturated solutions while for more dilute solutions it is necessary to have an excess of CrOa to maintain equilibrium. t

The mean proportion mentioned above of 2 mol. NaOH to 5 mol. CrOa has been found suit:-

able for solutions saturated with a third substance.

Moreover it has been established that the particular efiect mentioned disappears completely if, in consequence of the lowering, of the relative content of chromic acid, the ratio is reached: 2 mol. NaOH to 3 mol. Cl'Os that is the base acid ratio for which the tetrachromate cannot exist.

The increase in the content of chromic acid evidently does not preclude the existence of the tetrachromate so thatrin this case the decrease of the quality of the bath as regards the particular effect in question is only noticeable in proportion to the excess of chromic acid employed, while the decrease of the content of chromic acid leads to an extremely definite disappearance of this efiect.

For obtaining a bath having the above quoted composition of 2 molecules of NaOH to 4 molecules of CIOa (which corresponds toa concentrated electrolyte) I can proceed either by adding to 131 grams of caustic soda, 660 grams of. chromic acid and 210 cc. of water; or by mixing 490 grams of crystallized dichromate of sodium (2H2O) with 330 grams of chromic acid and 180 495 grams of chromic acid and evaporating until 57 grams of water have disappeared.

For realizing the proportion of 2 molecules of NaOH to 6 molecules of ClOs (which corresponds to a diluted electrolyte), I can proceed either by adding to 40 grams of caustic soda, 264 grams of chromic acid and 696 grams of water; or by adding to 163 grams of neutral chromate of sodium 10H2O), 210 grams of chromic acid and 627 cc. of water; or finally by mixing 123 grams of dichromate of sodium (2H2O), grams of chromic acid and 712 cc. of water.

In any of the above cases, the solutions are to be maintained at a temperature below 40 C.

The influence of the temperature constitutes a quite characteristic proof of the fact that sodium tetrachromate alone participates in the specialeffect in view. It has been established experimentally that with solutions saturated with third substances and for temperatures exceeding 30 C. there is a rapid diminution in the possibility of subjecting the solutions to high loads, the disappearance of the effect being complete at 40 C. Now the effective limit of stability of sodium tetrachromate is about 40 C., that is to say, that the tetrachromate no longer exists beyond this temperature.

By reason of the facts observed experimentally in the case of solutions of chromic acid containing sodium hydroxide, there may be attributed with certainty to sodium tetrachromate a preponderating part in the course of the electrolysis.

The deposit of chromium obtained with the maximum current density attains the hitherto unknown value of 3'7 of the theoretical value (at'25 C.) The evolution of hydrogen remains very slight, a great part of the energy previously utilized in the evolution of hydrogen now appearing to be employed in obtaining the chromium deposit. The appearance of the chromium deposit obtained is quite different from that which is obtained at similar temperatures in ordinary baths. For current densitiesof 250-1000 amp. per dm the colour is definitely yellowish; it becomes bluish for current densities of less than 250 amp. down to the limit of 3 amp. per dm below which the deposit no longer forms. The passage from the bluish to the yellowish coloration is produced with a small variation of current density. Beyond the upper limit of 1000 amp. (for baths containing about 25% of CrOs) the colour tends towards grey and towards about 3500 amp. a sort of dendritic arborization appears.

Between the current limits of 5 and 500 amps. per dm no tendency to the formation of excrescences has been observed, the deposits are absolutely smooth but not brilliant. Their subsequent polishing is, moreover, most simple. The deposits with a yellowish coloration possess, particularly for more dense layers, a noteworthy elasticity, while the bluish deposits resemble as regards hardness the chromium plating at present known. It results from this that if the process is used with the maximum admissible charge, there is formed on'an object comprising doubly curved surfaces a thick coat of yellowish chromium on the convex portions of the object, while the concave portions still receive a current intensity sufficient for complete covering at least with a bluish coating of chromium.

The coeflicient of dispersion in depth according to Pfanhauser has been found to be above 0.5.

The presence of foreign anions in the electrolyte only becomes harmful when they hinder the formation of tetrachromate but in any case they are not useful or necessary. Chromic acid produced by anodic oxidation of metallic chromium, that isto say, free from sulphuric acid (except for traces on analysis) has given results at least as good as those of a 9 commercial chromic acid. The metallic hydroxides, for example, chromic or ferric hydroxide, produce by their combination with chromic acid a relatively rapid diminution of the quantity of chromic acid available so that the conditions of existence of the tetrachromate tend to disappear; for this reason the bath must frequently be freed from these impurities.

As regards the mechanism itself of the electrolysis of an electrolyte containing tetrachromate, the reaction differs according to the current density. It only attains its typical qualities lhigh yield, yellow coloration of the chromium) for the most elevated current densities while, for medium or weak current densities, the yield and also the coloration of the chromium rather approach the known conditions.

The general equation for the electrolysis is:- For 1000 amp. dm

2C1'O3+33H+846Ah:2C-r+6H2O+10.5H2

coloration of the chromium: yellowishYield: 37% of the theoretical yield;--for 250 amp. dm z Coloration of the chromium: white-Yield: 37 of the theoretical yield.

For 200 amp. dm z -coloration of the chromium: whiteYicld 30% of the theoretical yield.

For 5 amp. dm z 8CrO3+60H+1600Ah= 2Cr +3C12O'3 151-120 15H2 --coloration of the chromium: bluish-Yield: 20% of the theoretical yield.

In these equations Ah signifies ampere hours.

These equationsshow clearly that the baths of tetrachromate only give their full capacity with highest current density and they explain why the appearance of the chromium is in this case different from that obtained by the known baths and with a weaker intensity.

The formation of the intermediate chromium range (chromates of chromium) is prevented the more, the higher is the cathodic pressure of hydrogen; the reduction of ClOs to Cr is in this case preferably produced directly. The deposited chromium then automatically takes the molecular structure of ClOa which determines among other things the coloration of the deposit.

For weaker intensities and above all, for ordinary electrolytes containing other weak acids, the reduction takes place principally on the chromic range. The bluish coloration of the chromium which is deposited in this case is due to the molecular structure of the chromium hydroxide. The hardness of the chromium, moreover, varies in a corresponding manner to the coloration.

The yellow chromium is more ductile than the blue chromium, the chromate structure being less dense than the chromic structure. Certain peculiarities are directly due to this fact.

With the exception of its tendency to the adsorption of hydrogen, the chromic chromium is definitely more inactive from the chemical point of view than, the chromate chromium, this latter being inactive as regards the adsorption of hydrogen but more sensitive to other influences and.

moreover, possessing in a certain degree the tendency to reaction of the CrO: or chromic acid.

When hardness and chemical resistance are required, the chromic chromium is superior to the other, which in its turn is'to be preferred when there is desired a certain ductility and chemical reactivity. This latter thus serves for chromium plating intended subsequently to be machined (rolling and cutting) or for objects subjected to considerable bending (wires).

On account of its chemical reactivity it has been found that the chromate chromium has retained the tendency which chromic acid likewise possesses, that is to say, the tendency to attack metals, i. e. in the metallographic sense, that is to say, of the formation of alloys with the support. The layers of chromium deposited in baths of tetrachromate adhere atomically to the metals of the base, above all in the case in which the structure of these latter is favourable.

For this reason, success is obtained with re markable ease in producing perfect alloys by simple heating of the metals chromium plated with chromate chromium. The production of alloys of chromium by this process constitutes the principal field of application of the chromate chromium. One can successfully transform chromium plated aluminium, for example, directly into an aluminium chromium alloy, by heating it simply at temperatures which are lower than the point of fusion of the aluminium. Iron and nickel can just as easily be converted into thei chromium alloys.

Even the zinc-chromium alloy which is difficult to obtain by other means can be thus produced.

As regards the technical working with baths of tetrachromate this is determined by the strong intensities necessary. The mean current density of 500 amp. dm is obtained for a resistance of'2.4 2 per cm. cm with a distance of 1 cm. between electrodes under 12 volts. The potential of cathodic separation is exactly 1.62 volts.

As the bath only attacks the metals weakly (even in the case of zinc), on account of the lack of free acid, direct chromium plating is the preferred operative method. By reason of the strong intensities employed, and of the necessity for not exceeding a temperature of 40 C. for the bath in order to satisfy the conditions of the invention (existence of sodiumtetrachrom'ate), it is absolutely necessary to cool the bath if a 2. Process as claimed in claim 1 in which thebath is a saturated solution in which the molecular proportion of NaOH with respect to CrOz is about 2 to 4.

3. Process as claimed in claim 1 in which the, bath is a dilute solution in which the molecular proportion of NaOH with respect to C1O3 is 2 to 6, in particular 2 to 5.

4.. In chromium plating effecting the elec-'- trolysis at a temperature below 40 C. with a bath of chromic acid containing sodium tetrachromate using a current density greater than 200 amperes per square decimetre.

5. In chromium plating effecting the electrolysis at a temperature below 40 C. with a bath of chromic acid containing sodium tetrachromate using a current density of 2004000 amperes per square decimetre.

6. In chromium plating effecting the electrolysis at a temperature below 40 C. with a bath of chromic acid containing sodium tetrachromate using a current density of 500 amperes per square decimetre with a. resistance of 2.4 a per cin/cm and a distance of 1 cm. between electrodes under 12 volts, the potential of cathodic separation being 1.62 volts.

7. A process of rapid electrolytic chromium plating which consists in: rendering the bath of chromic acid effectively responsive to a high amperage current by adding to the bath a substance of the group comprising sodium dichromate, neutral sodium chromate and sodium hydroxide in such quantity as to form sodium tetrachromate, maintaining said bath below 40 0.,

and utilim'ng a current density 'of substantially 200 amperes or more per square decimetre.

OSCAR BORNHAUSER. 

